Rapid rotation of Gondwana in Cambrian?

The land mass that contained what is now South America, Africa, India, Arabia, Antartica, Australia and a part of Southeast Asia rotated about 60% very rapidly during the early part of the Cambrian period about 525 million years ago, according to a study published today in Geology (abstract; full article behind paywall). Ross N. Mitchell, David A.D. Evans and Taylor M. Kilian, all of the Department of Geology and Geophysics at Yale, studied the paleomagnetic record of the Amadeus Basin in central Australia.

Around the beginning of the Cambrian period (~542 mya) the various land masses amalgamated into the supercontinent precursor called Gondwana. This land mass would later attach with masses that are now North America, Europe and Siberia to form the supercontinent Pangaea. (Pangaea would later break up beginning in the late Triassic period into Gondwana and Laurasia.) The Cambrian was the period of the Paleozoic era during which a large array of the first complex organisms (built on a wide array of body plans) appeared in the fossil record — the so-called Cambrian explosion.

The paleomagnetic record from the Amadeus Basin in Australia (marked by the star) indicate a large shift in some parts of the Gondwana supercontinent relative to the South Pole. (Illustration: Ross Mitchell/Yale University)

Based on their measurements of the rocks’ magnetization, the authors postulate that Gondwana landmass underwent a rapid 60-degree rotational shift, with some regions attaining a speed of at least 16 (+12/-8) centimeters per year. This speed is about four times the maximum speed of the fastest continental drift today.

In the press release by Yale today, lead author Ross Mitchell, a Yale graduate student, said that the shift could either be the result of plate tectonics (the individual motion of continental plates with respect to one another) or “true polar wander,” in which the Earth’s solid land mass (down to the liquid outer core almost 3,000 km deep) rotates together with respect to the planet’s rotational axis, changing the location of the geographic poles.

The calculations depend on the assumption that the entire land mass had been assembled at the time of the magnetism measured, about 525 million years ago. The paper concludes that the observations can be explained either by nonuniformitarian plate tectonics or an episode of rapid true polar wander. Either event had to have occurred during the Cambrian “explosion” of animal life. The press release says: “Mitchell and his team suggest that the rates of Gondwana’s motion exceed those of ‘normal’ plate tectonics as derived from the record of the past few hundred million years. ‘If true polar wander caused the shift, that makes sense. If the shift was due to plate tectonics, we’d have to come up with some pretty novel explanations.'”

The calculation would have Brazil moving from under the South pole to the tropics in a wide arc relatively quickly (more quickly than plates have every been observed to move). Of course, that depends on the South American mass having already been assembled to Gondwana (which the authors do not seem to question.) “Such large movements of landmass would have affected environmental factors such as carbon concentrations and ocean levels, Mitchell said.” This obviously would have had a major effect on the development and extinctions of the various organisms which appeared during the Cambrian explosion.

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